Alumina having an excellent insulating property, heat resistance and mechanical strength has been widely used as an electrically insulating material. However, the sintering temperature of alumina is 1300.degree. to 1600.degree. C. This high firing temperature leads to a high cost of production.
In addition, boards, etc. have been made of high-melting metals, such as W and Mo-Mn as a conductive material. However, such conductive metals have high electrical resistance, and therefore, the speed of transmitting signals is slow.
In Japanese Patent Application Nos. 27124/1985 and 27125/1985, ceramic materials which can be fired at temperatures of not higher than 1,000.degree. C. and on which a low-electric resistance conductor such as Ag, Ag-Pd, Au and Cu, can be attached and baked were studied. The results of these studies yielded a material composed of SiO.sub.2 -CaO-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 and zircon and a material composed of (SiO.sub.2 -CaO-B.sub.2 O.sub.3 -Al.sub.2 O.sub.3), respectively.
Crystallized glass has a relatively high mechanical strength as a material onto which a low-electric resistance metal is baked, but is of high cost. A sintered ceramic body is inexpensive, but has the disadvantage of low strength. The ceramic compositions described in the above-cited Japanese Patent applications are improved to some extent, but it is desired to develop a more improved ceramic composition.